1. Field of the Invention
The present invention relates to vapor pressure and more specifically to an apparatus and method of measuring the vapor pressure of multi-component liquids.
2. Description of the Related Art
Prior to the 1940s, light hydrocarbons and hazardous gases byproducts were considered undesirable in various petrochemical products. These undesirable byproducts were removed by venting them into the atmosphere during the processing of various hydrocarbons. As the public became increasingly concerned about the environmental and safety issues related to the venting of these gases, government agencies began to regulate the environmental emission of such gaseous byproducts of chemical processes.
In order to control the emission of these gaseous byproducts, the petrochemical industry needed to be able to determine the circumstances governing the release of such gaseous byproducts from multi-component petrochemical liquids. Since the vapor pressure of liquids is proportional to the pressure at which liquids will start to flash out of the liquid phase into a gaseous phase, the petrochemical industry was forced to look for simple and practical procedures to determine the pressure at which the first gas bubble was released from a multi-component fluid (referred to as the “bubble point” in the oil industry and the “boiling point” in the chemical industry).
From the 1930s to the late 1960s, vapor losses were primarily predicted by the Reid vapor pressure (RVP) method in accordance to ASTM-323 standards. In the 1950s and 1960s, it became obvious that there were errors in predicting vapor pressures in accordance with the ASTM-323 standards. As a result, the American Petroleum Institute (API) developed Monograph 2517 setting out corrections for minimizing errors in predicting vapor pressures. However, over the years it became apparent that even these corrections fell short of the industry and environmental requirements. Thus, the API Monograph 2517 has become obsolete over the past few years. Then in the 1980s, various products and testing procedures were developed to predict the vapor pressures of multi-component liquids at specified temperatures. Several of these processes and products are described in U.S. Pat. Nos. 5,637,791 (Alonso), 5,499,531 (Henderson), 5,889,202 (Alapati), 4,905,505 (Reed-1) and 5,172,586 (Reed-2).
Alonso utilizes an online analyzer to continuously determine the vapor pressure of a multi-component liquid. The online analyzer measures the highest pressure of the liquid at a given temperature at which the lightest component of the liquid starts to flash. Alonso's analyzer utilizes an upstream aeration or density measurement means and a pressure reducing means (such as capillary tubing) to reduce the pressure to the vapor pressure of the liquid without creating pressure recovery. However, this method has proved unsuitable for it is unable to control the pressure of the liquid as necessary for really accurate determinations of vapor pressure.
The Henderson patent discloses a system and method for calculating the composition of a liquid hydrocarbon using an iterative mathematical algorithm. The Henderson system is a time consuming process and requires expensive equipment, such as a chromatograph and a computer, to calculate the fluid composition and the liquid vapor pressure. Yet, the Henderson system does not provide for the direct measurement of the vapor pressure of a fluid.
Alapati describes a continuous direct analysis of an influent stream of liquid hydrocarbons through a liquid/gas separation chamber, a constant flow liquid influent means, influent and effluent flow metering means, and means for sensing the composition of the gaseous effluent. However, the accuracy of the Alapati equipment has not been verified.
Reed (Reed-1 and 2) determines true vapor pressure of a liquid composition by using a piston and cylinder apparatus to trap a liquid sample, then expanding the chamber volume in multiple steps and recording the multiple pressure drops. A resultant pressure is determined using the least square method of approximation. Then the result and pressure values are extrapolated to determine the true vapor pressure using a straight-line approximation. The apparatus does hot measure the vapor pressure directly and, therefore, is not as accurate as desired.
Even today, many engineers and industry personnel are still confused whether RVP by ASTM-323 procedures is reported in absolute or gauge pressure readings. Today almost all of the results are reported as True Vapor Pressures (TVP) and not RVP, thereby eliminating the conversion from RVP to TVP and much of the prior confusion.
There is a continuing need for a simple and rapid method and apparatus for accurately determining the bubble-point pressure at a constant temperature of a multi-component liquid.